Babel, JSX, and Build Steps

From where this @sample-stack/platform-browser comes in packages/sample-platform/browser/src/containers/Counter.tsx?

We use lerna for monorepo. For example all packages starts with @sample-stack are either under packages or pacakges-modules directories.

Why you have used hot?

It is used for hot reloading. Checkout out React Hot Loader

What is use of rehydrate?

It is used for SSR(server side rendering). Server generates initial browser page along with state(redux or apollo-client or css) and that state will be used in the browser to rehydated during browser rendering so the client state don't start from initial values.